1. Field of the Invention
The present invention is in the field of impact devices and relates more particularly to an impact fastener device of the type employed in the air conditioning and heating fields to secure batts of insulation to the surfaces of ducts.
2. The Prior Art
It is conventional practice in air heating and cooling systems to minimize heat losses by attaching insulative material to duct components. Attachment may be effected by adhesive connections, typically supplemented by metallic fastener members extending through the insulation and secured to the underlying duct. The supplemental metallic connectors may be secured by welding, as in U.S. Pat. No. 4,482,795 of Nov. 13, 1984; U.S. Pat. No. 4,429,209 of Jan. 31, 1984; U.S. Pat. No. 3,701,874 of Oct. 31, 1972; and U.S. Pat. No. 3,624,340 of Nov. 30, 1971, by way of example.
An alternate means of attaching metallic fasteners is by way of an impact connected unit. Representative examples of such units are disclosed in U.S. Pat. No. 4,003,286 of Jan. 18, 1977 and U.S. Pat. No. 3,095,777 of July 2, 1963.
It is generally recognized that the most efficient connection is effected by welding type fasteners. Such fasteners are highly resistant to dislodgement from the metal when subjected to pulling stresses and/or to the stresses of laterally applied forces which tend to bend the fasteners.
Fasteners of the impact type heretofore known, while reasonably resistant to separating forces exerted in the direction of the axis of the attached pin, offer limited resistance to forces exerted in a direction normal to the longitudinal axis of the pin. Additionally, impact applied fasteners heretofore known over the passage of time have tended to work loose when subjected to the continuous vibration normally experienced in air ducts.
A particular shortcoming of impact fasteners of the type hereinabove set forth has resided in the fact that the stability of the fasteners is, in large measure, dependent upon the gauge or thickness of the metal in which they are secured, and the force of the impact employed in the attaching operation.
Referring more particularly to the fastener devices as set forth in U.S. Pat. Nos. 4,003,286 and 3,095,777, it will be perceived that the same are comprised of a blunt metal penetrating point which, upon impact, embeds into the metal and surrounds a ball of increment thereof. Axially spaced from the fastener point there is provided a pair of laterally extending stop portions. Where the fastener is driven into a thick gauge metal in such manner that the fastener is embedded to a depth whereat the stop portions engage the surface of the metal, a reasonably stable connection between fastener and metal is achieved at least against forces applied normal to the axis of the shank in the plane defined by the stop portions. If a force is applied normal to such plane the connection offers little resistance to stresses and the fastener will readily break free from the sheet metal.
A greater problem is confronted when an attempt is made to drive conventional impact fasteners into thin gauge metal. If, on the one hand, the fastener is driven sufficiently far that the stop portions engage against the surface of the metal, there results a complete severing by the blunt point of an increment of the metal, with resultant low resistance to a pulling force exerted on the fastener. If, on the other hand, the fastener is driven into thin sheet metal stock in such manner that the point embeds but does not puncture the metal (the desired result), the stop portions will be spaced away from the surface of the metal. An impact fastener attached in this manner, while affording reasonable resistance to removal in the direction of the axis of the fastener, affords extremely limited stability against lateral dislodging forces since the stop portions are not braced against the surface of the sheet metal.
From the foregoing it will be perceived that impact fasteners heretofore known are dependent for security of attachment on the thickness of the sheet metal to which they are secured, and it is desirable to provide a range of fasteners correlated to the thickness of the sheet metal. Additionally, the stability of known fasteners to resist laterally applied forces is highly dependent upon the force with which the fasteners are attached.